I think our aerials are better too. Not sure if this is the case, but it seems like a lot of Rosalina's aerials are clunky and punishable. I suppose we can get rid of Luma efficiently, but the very presence of Luma throws off our game more so.
I think in the case of Sheik, it because a question of how perfect your spacing is. I mean, you always want to go for the perfect spacing, but it really matters in this MU. Her strings can really mess us up, but I feel like it can be hard to catch us in them. The only aerial that we have to really dent them, imo, is fair. It tacks damage on fast, has a largely disjointed hitbox, and that knockback pushes far. What you should focus on against Sheiks is dissecting their habits and punishing them. Don't approach. Grab combo as much as you can, even if it's stale. Someone should probably disagree with me here, because I kind of feel like I'm talking out of my ass here.